It is well known that a quadrupole magnetic field influences the beams generated by the three in-line guns by moving the outer beams in opposite directions and that a six-pole magnetic field moves the outer beams in the same direction.
British Patent Specification No. 1466732 (U.S. Pat. No. 3,902,145) describes the use of quadrupole and six-pole fields in converging in-line beams. The quadrupole unit consists of two concentric magnetic rings each with four poles distributed about its periphery. The two rings are relatively rotatable and can also be rotated together as a unit. The strength of the quadrupole field is adjusted by adjusting the relative angular position of the two rings. The direction of the quadrupole field is determined by the angular position of the unit. A similar arrangement is provided for adjusting the six-pole field.
British Patent Specification No. 1532462 (U.S. Patent No. 4,045,754) describes how, for more accurate convergence, the six-pole two concentric magnetic ring arrangement should be replaced by two such arrangements each mounted asymmetrically of the in-line beams. The specification also describes how each six-pole structure can be replaced by a twelve-pole electromagnet connected as two six-pole electromagnets angularly separated by 30.degree.. The disadvantage of this proposal is that very careful adjustment is required not to lead to shift of the center beam. This excludes use as a dynamic convergence unit.
U.S. Pat. No. 4,027,219 shows a static convergence unit formed from a magnetic strip upon which coils have been wound. No means are shown to maintain the position of the coils during or after manufacture.